2003 Johnson ST 55 HP WRL 2 Stroke Commercial Service Manual, PN 5005483, Page 103Get this manual
FUEL SYSTEM COMPONENTS
Fuel Pump
The fuel pump is diaphragm-displacement pump type operated by changes in crankcase pressureThe pump lifts fuel to the outboard and supplies fuel to the carburetor
Carburetors
A carburetor meters and distributes the fuel consumedIt controls engine speed and power output by varying the intake charge densityTo react correctly to wide range of operating conditions, the carburetor has multiple fuel mixing circuits
The diaphragm of the pump is driven by crankcase pressure and vacuumA pulse hose connects the pump to pulse fitting that threads into the front of the cylindercrankcase